Accumulation of the 25K casein mRNA in rat mammary explants requires insulin, gluccocorticoid and prolactin. We reported last year that prolactin is essential for transcription of the gene, and markedly increases the half-life of the transcript. We reported that insulin, too, is essential for the transcription, but has virtually no effect on the stability of the transcript. Now we report "early" and "late" effects of glucocorticoid-depletion on this system. Culture of mammary explants from mid-pregnant rats for a total of 6 days in the presence of insulin and prolactin, in the absence of exogenous glucocorticoid, leads to no decline of transcription of the 25K casein gene relative to that observed in the presence of glucocorticoid. However, in the absence of exogenous glucocorticoid the half-life of the mRNA is reduced from20-24 h to less than 1 h. Longer culture in the absence of glucocorticoid leads to complete loss of transcription of the casein gene, although total transcription is unaffected. Developmental changes in the carrier-mediated transport of glucose by mouse mammary epithelial cells are under current study. The initial rate of this transport (using 3-0-methylglucose) increases about 40-fold as the donor animal progesses from the adult virgin to the 10-day lactating state, and reverts to the low, virgin-like value during involution. The increment in the rate during the interval between mid-pregnancy and 2-days of lactation is about 5-fold. CUlture of mammary explants from mid-pregnant mice for 3 days also leads to a 5-fold enhancement of the rate of carrier mediated glucose transport by the epithelial cells isolated from the explants. This stimulation requires physiological concentrations of insulin or IGF I (EGF is ineffective), in addition to glucocorticoid and prolactin. Although insulin can acutely elicit a small (20-50%) increment in the rate of transport, the much larger developmental enhancement produced by the hormone in culture is considered to be more relevant physiologically.